With the recent expansion in the use of computers and computer terminals it is not unusual to have a large number of computers and terminals within a limited local area. It is very desirable to couple these units together to allow sharing of resources and permit a single terminal to access a plurality of other computers or terminals. One way of accomplishing this is the local area network (LAN). A local area network provides switching and data distribution for the transmission of information and allows computers to communicate with each other. LAN's further allow any single terminal to have access to a multiplicity of computers and peripheral equipment.
Local Area Networks, such as are used, for example, in computer communications, are well known and described in U.S. Pat. No. 5,041,963, entitled, "Local Area Network with an Active Star Topology Comprising Ring Controllers having Ring Monitor Logic Function", by Ebersole et al., issued Aug. 20, 1991; U.S. Pat. No. 4,998,247, entitled, "Active Star-Configured Local Area Network", by Irvine-Halliday et al., issued Mar. 5, 1991; U.S. Pat. No. 4,982,400 entitled, "Ring Bus Hub for a Star Local Area Network", by Ebersole, issued Jan. 1, 1991; U.S. Pat. No. 4,825,435, entitled "Multiport Repeater", by Amundsen et al., issued Apr. 25, 1989; U.S. Pat. No. 4,872,158, entitled, "Distributed Control Rapid Connection Circuit Switch", by Richards, issued Oct. 3, 1989; U.S. Pat. No. 4,787,082, entitled, "Dataflow Control Arrangement for Local Area Network", by Delaney et al., issued Nov. 22, 1988; U.S. Pat. No. 4,674,085, entitled, "Local Area Network", by Aranguren et al., issued Jun. 16, 1987; all of the foregoing being incorporated by reference.
The stations or nodes of a local area network may be configured in a variety of shapes, such as, for example, as a ring or a star. In a star-configured or local area hub network, multiple, discrete, remote stations are coupled through a central site or station, termed a hub station. The hub station follows a particular or predetermined signaling protocol to establish communications and to determine the order in which remote stations are permitted to transmit data over the network in the form of signals, such as electrical or optical signals. In one such signaling protocol, termed round robin, each of the remote stations is separately polled for transmissions. Likewise, in a round robin signaling protocol, only one station may transmit electrical signal data over the network at a time; that is, only one station may have controlling access to transmit a packet on a memory bus.
According to a ring network topology, transmission is from node to node around a closed loop and each node may alter the data passing therethrough. Each data processing device is connected to a separate node and intercepts only data messages specifically directed to a node. Since the data flows through each node and since the nodes are distributed, no central node may be separately secured and hence, the security and privacy of a ring network topology is somewhat less than that of the star network topology. Additionally, all nodes are in one closed loop and, failure of a single node may render the whole ring network inoperative. The closed loop topology also limits flexibility in some arrangements where adding new nodes or data processing stations results in the loss of data.
As with the star topology discussed above, hubs in a ring topology must follow a particular or predetermined signaling protocol to establish communications and to determine the order in which remote stations are permitted to transmit data over the network in the form of signals. The round robin signaling protocol discussed above is one such protocol and is applicable to a ring topology in the same manner it applies to a star topology.
Data transmission in a bus network topology is typically broadcast from one source to all other devices on the same bus, but is normally only accepted by the device to which it is specifically addressed. Individual data processing devices are programmed to recognize data messages addressed to or intended for them as they pass them by on the bus. The reliability of the bus network topology, in terms of network node failure, is greater than that of the ring network topology, although a break in the bus may be catastrophic. There is also greater flexibility in adding new data processing devices to the system than is normally possible with the star topology since no wiring reconfiguration is required. Although a round robin protocol is generally not applicable to a bus topology, it might be applicable if one hub is designated as the controlling hub. If this were the case, the controlling hub would be programmed with logic to govern the remaining hubs in accordance with the round robin protocol.
As disclosed and described in Draft for Standard Information Technology Local and Metropolitan Networks--Part 12--Demand Priority Access Method and Physical Layer Specification, P802.12, dated March 1994, herein incorporated by reference, a round robin protocol has been proposed to the IEEE to be employed in the local area networks transmitting data in the form of electrical signals 100 megabits per second, termed 100 Base VG. A round robin protocol provides advantages over other known network protocols, such as the protocol employed by the IEEE standard 802.3, also known as CSMA/CD (Carrier Sense Multiple Access with Collision Detection). The advantages associated with the round robin protocol are especially applicable for time sensitive multimedia communication tasks. Specifically, in multimedia applications it becomes desirable to prioritize or control access to the media or communication networks provided to different remote stations. The prioritized or controlled access permits more important communications tasks to obtain access to the network earlier than the less important tasks.
Several different media can be used to carry local area network communications. Considerations regarding network topology, maximum distance between nodes, volume of information to be transmitted, and speed of transmission are critical in a selecting particular communications medium. Physical limitations such as plenum, conduit sizes, and routing plans in the building also affect the choice of the medium. Finally, for some network topologies, user accessibility to the bus for passive tapping is also extremely important.
FIG. 1 is a schematic diagram of one embodiment of a conventional local area star-configured or hub shaped network. As previously suggested, in a local area hub network, the remote stations 10 may be arranged in a star-shaped network, or in a star configuration, in which a central station acts as a hub 12 for the remote stations 10. Thus, a hub or hub station may be used to transmit a signal packet, such as an electrical signal packet, from one remote station to another remote station, or multiple remote stations, since all of the remote stations are in direct communications with, or directly coupled to, the hub. Remote stations may comprise, for example, a data terminal or other computer-related equipment, as described in the aforementioned Demand Priority Access document.
In the context of the invention, the term "packet" refers to a complete and discrete grouping of data in the form of signals, typically digital signals, for transmission between stations. Thus, for example, a packet may comprise digital signals to be transmitted. Typically, packets include a start of frame delimiter (SFD), and an end of frame delimiter (EFD). Likewise, as described on page 4-3 of the aforesaid Demand Priority Access document, and as illustrated in FIG. 2, a packet may further include binary digital signals, or bits, representing, for example, a destination address (DA), a source address (SA), the length of the packet (L), the data to be transmitted (DATA), and a frame check sequence (FCS) for signal error checking.
In the network illustrated in FIG. 1, packets, such as electrical signal packets, transmitted between two remote stations must pass through the hub. Depending on the particular network, the hubs typically have the capability to perform and recognize a signaling protocol (often termed "handshaking"), the capability to identify and extract data from an electrical signal packet, the capability to store data in the form of electrical signals, the capability to perform signal error checking, and the capability to perform destination address matching, such as described in the aforementioned Demand Priority Access document.
For a local area hub network, it often becomes desirable to increase the size of the network without substantially degrading signal transmission performance; that is, it is desirable for the network to be extendible. However, as local area hub networks increase in size, the hardware for communications between remote stations of the network becomes increasingly complex. For example, technological limitations on the manufacture of integrated circuit chips typically restrict the number of ports that may be fabricated on one chip. Specifically, in any integrated chip implementation of a multiple-port LAN hub the number of user ports is usually limited to between six and twelve per single hub integrated chip due to physical limitation. Thus, for a large local area hub network having, for example, tens or hundreds of stations, communications between the remote stations directly linked or coupled to one hub station in the network may need to be shared among several, discrete devices or chips. Additionally, since all the network traffic must go through a specific hub, the bandwidth available to each port decreases as more ports are added to the hub. Further, the problem of performing or controlling round robin polling between these discrete devices also becomes more complex.
One way to overcome the limited number of ports available on a signal hub is the coupling of hubs to form a local area hub network consisting of multiple hubs. For such a local area hub network, the hubs should be coupled and signals must be communicated between the hubs so that the performance of the round robin signaling protocol is not substantially degraded. Furthermore, it is desirable not to increase the complexity of the hardware unduly and to exhibit low pin or port overhead by reducing the number or maintaining a relatively low number of ports needed to accomplish satisfactory operation.
In view of the foregoing discussion a need exists for an extendable local area hub network which efficiently, reliably and functionally increases the number of ports available on a local area hub network. The present invention provides such a network.